The development of active nanostructures, capable of performing a function or executing a specific task, is currently a major focus of research efforts in nanotechnology. One already highly successful nanodevice paradigm is the nanosensor: a designed nanostructure, which when addressed either optically or electrically, responds by providing information about its local environment through its optical or electrical response. All-optical nanosensors hold exceptional promise as embeddable nanodevices that could potentially be used to probe a variety of exceedingly complex environments, such as individual living cells or the human body, in a virtually noninvasive manner. In addition to passive and active functions, there is a third, “informative” role, for which metallic nanoparticles are particularly well suited, and which enables nanosensor development. Metal nanostructures, when excited on their plasmon resonance, give rise to high intensity electromagnetic fields at their surfaces which can be harnessed for chemically specific sensing functionalities. Metallic nanoparticles can be designed to enhance the spectroscopic response of molecules bound to, or in close proximity of, their surfaces, an effect known as surface enhanced Raman scattering (SERS). Detailed local chemical information can be retrieved from the spectrum of the inelastically scattered light from the nanoparticle complex. Thus, it would be desirable to develop an all-optical nanosensing device.